A major criticism of glass articles is based on weight. This is particularly true of ophthalmic lenses where comfort of the user is very important.
One approach to reducing the weight of glass lenses is to make the overall lens thinner. Most glass lenses, particularly those with a minus prescription, are about 2.2 mm. thick at the thinnest point on the lens. Reducing the lens thickness to 1.0 mm. at the thinnest point reduces the lens weight by up to 55%, depending upon the lens prescription. However, such a thin lens by itself may not meet the FDA requirements for impact resistance.
The weight disadvantage of glass has led to the advent of plastic lenses. However, plastic materials are generally less scratch resistant than glass. Also, plastic materials do not offer reliable photochromic characteristics.
A further criticism of glass arises from its brittle nature, and the potential danger created by glass shards when a glass article breaks. This problem has been countered in vehicle windshields by resorting to a laminated product having an inner plastic film or layer. In that product, glass shards tend to remain bonded to the plastic inner layer after fracture.
Numerous proposals have been made to produce a glass-plastic, composite, ophthalmic lens that combines a thin, glass lens member with a plastic film layer. The usual proposal for minimizing weight is to apply a plastic film on the back surface of a glass lens member. The front glass surface, where abrasion is most likely to occur, provides scratch resistance.
U.S. Pat. No. 5,073,423 (Johnson et al.) discloses a system by which thinner, lighter weight, ophthalmic lenses can be produced while still meeting the appropriate impact requirements. This approach involves pressing a tough, abrasion-resistant, polymeric film to the back surface of a finished lens. A mechanical presshead with a silicone applicator head is used for pressing. Heat may be used to improve adherence.
A tough, abrasion-resistant film, such as a polyurethane, applied and adhered to the back concave surface of a lens, restrains glass fragments from striking the wearer. In a manner similar to a laminated safety glass, or an automobile windshield, the film reduces the risk of glass fragments coming into contact with the wearer. Typically, most, if not all, of the glass fragments remain bonded to the film even when a glass lens component is fractured.
The method proposed in the Johnson et al. patent offers advantages over other possible techniques for applying a plastic film, such as autoclaving. The process is quite simple, and the cost of the equipment required is relatively small. Hence, the method is well adapted to optical lab practice because the composite lens can be produced on-site, and a relatively small capital investment is required.
However, when attempts were made to implement the procedure of the Johnson et al. patent, certain problems were encountered. In particular, optical defects occurred in the film; also, problems with optical quality, and with film adhesion, occurred in changing from production of one lens prescription to another. The present invention represents an improved method that is designed to avoid, or correct, the indicated problems.